The invention is directed to a composition and process for the electrolyte deposition of silver and silver alloy coatings out of an electrolyte that contains or consists of a salt melt.
Because of their high electrical conductivity and good resistance to oxidation silver and silver alloys are among the most important electrical contact materials in the electrolyte art. Additionally the price of silver is relatively low compared to other noble metals.
Limiting the use of silver is the low resistance of silver in sulfur containing atmosphere. There occurs through the silver sulfide tarnish layer formed thereby not only a discoloration of the surface but also an increase of the electrical contact resistance of these coatings.
In the energy art where high current flows at relatively high contact forces these sulfidic tarnish coatings cause only slight disturbance.
It is different in the weak current art where it is important that at low contact strengths the contact resistance remains small and constant.
Metals which form alloys with silver that increase essentially are the platinum group of metals, e.g. platinum, palladium or ruthenium, as well as gold. To be sure for example a metallurgical melt produced silver-palladium alloy is only resistant to sulfur if the palladium component is more that 30 weight %. Such alloy coatings also can be applied galvanically but these silver alloy coatings have the disadvantage if they are deposited in a wet galvanic method that to reach the same resistance to sulfur the portion of alloying metal, thus e.g. palladium, according to experience must be higher than in the melted alloys.
Therefore it was the problem of the present invention to develop a process for the electrolytic deposition of silver and silver alloy coatings by which sulfur resistant coatings could be produced with the least possible portion of alloyed platinum group metals and/or gold.